The invention disclosed herein relates generally to a method and means for providing temperature compensation for elements whose sensitivity is a function of electric current therethrough, and more particularly to a method and circuitry for compensating for the temperature dependence of the sensitivity of Hall effect elements and other elements with similar characteristics.
The Hall effect is being used to an increasing extent for measuring, controlling and regulating purposes. For example, in combination with a voltage amplifier, a Hall effect element may be used as a stable signal generator or as a switch or boundary value indicator which functions without physical contact. The theory of operation of a Hall effect generator is well-known. The effect may be generally described as follows. If a block or sheet of suitable material having orthogonal axes x,y and z is fitted with a pair of input electrodes such that a current flows along the x axis, and if a magnetic field is passed through the material generally parallel to the y axis, then a Hall voltage will be produced across the material in the direction of the z axis. A pair of output electrodes may be connected to the material such that the Hall voltage can be applied to an output circuit.
Materials suitable for Hall effect generators generally exhibit large increases in resistance with increasing temperature. The same is true for certain other materials sensitive to magnetic fields, such as permalloy. Thus, a fixed voltage applied to a Hall or other similar element results in a current therethrough which decreases rapidly with increasing temperature. Partially as a result of the decreasing current and partially as a result of the greater portion of the Hall voltage which is dropped internally the output voltage of the Hall element also decreases. This large negative temperature coefficient is manifested as a reduction in sensitivity as temperature increases. In many applications a large variation in sensitivity cannot be tolerated, or is at least undesirable. In such applications it is necessary to provide means for compensating for the temperature dependence.
A variety of circuits and apparatus have been devised for compensating for the temperature dependent characteristics of a Hall element. For example, British Pat. No. 1,247,955 discloses Hall effect apparatus in which it is attempted to provide temperature independent sensitivity, in part by maintaining a constant current through a Hall element by connecting large value resistors in series therewith. The apparatus also includes an output circuit in which the internal resistance of the Hall element forms a part of a feedback network for a differential amplifier.
Maintaining a constant current through a Hall element may not be feasible in some situations. Also, the Hall element in British Pat. No. 1,247,955 is described as having decreasing resistance with increasing temperature which is generally opposite to the temperature response of known Hall effect materials. Operation of the output circuit appears to depend on this unconventional temperature dependence. Finally, the Hall element and compensation circuit disclosed in the patent are not well adapted to manufacture by the most common present monolithic integrated circuit fabrication processes.
A mechanical arrangement for providing temperature compensation for a Hall effect device is shown in U.S. Pat. No. 3,435,332 issued to N. Kurdlya on Mar. 25, 1969. In this arrangement, the magnetic structure which provides flux for the Hall element is mounted in apparatus which varies the air gap and, hence, the flux concentration with temperature so as to compensate for variations in Hall element sensitivity. This arrangement is mechanically complex, and therefore also undesirable for many present applications.
The applicants have provided a unique temperature compensation method and circuit which is simple and applicable to various elements of the type whose sensitivity is a function of electric current therethrough. It is also readily manufactured with elements, such as Hall elements, in monolithic integrated circuit form by common integrated circuit fabrication processes.